<PAGE>
Exhibit 5
BINGHAM DANA LLP
150 Federal Street
Boston, MA 02110
November 8, 2000
Cubist Pharmaceuticals, Inc.
24 Emily Street
Cambridge, Massachusetts 02139
Re: Registration Statement on Form S-8 Under the Securities Act of 1933,
as amended
Dear Sir or Madam:
We have acted as counsel for Cubist Pharmaceuticals, Inc., a Delaware
corporation (the "Company"), in connection with the Company's Registration
Statement on Form S-8 to be filed with the Securities and Exchange Commission on
November 8, 2000 (the "Registration Statement").
The Registration Statement covers the registration of an additional
1,000,000 shares of common stock, $0.001 par value per share, of the Company
(the "Shares"), which are to be issued by the Company upon exercise of stock
options issued or to be issued pursuant to the Cubist Pharmaceuticals, Inc.,
Amended and Restated 1993 Stock Option Plan (the "Plan").
We have reviewed the corporate proceedings of the Company with respect to
the authorization of the Plan and the issuance of the Shares thereunder. We have
also examined and relied upon originals or copies, certified or otherwise
identified or authenticated to our satisfaction, of such agreements,
instruments, corporate records, certificates, and other documents as we have
deemed necessary or appropriate as a basis for the opinions hereinafter
expressed. In our examination, we have assumed the genuineness of all
signatures, the conformity to the originals of all documents reviewed by us as
copies, the authenticity and completeness of all original documents reviewed by
us in original or copy form, and the legal competence of each individual
executing any document. As to all matters of fact (including factual conclusions
and characterizations and descriptions of purpose, intention or other state of
mind) we have relied entirely upon certificates of officers of the Company, and
have assumed, without independent inquiry, the accuracy of those certificates.
<PAGE>
-2-
We further assume that all Shares issued upon exercise of options granted
or to be granted pursuant to the Plan will be issued in accordance with the
terms of such Plan and that the purchase price of the Shares will be greater
than or equal to the par value per share of the Shares.
This opinion is limited solely to the Delaware General Corporation Law, as
applied by courts located in Delaware, the applicable provisions of the Delaware
Constitution and the reported judicial decisions interpreting those laws.
Based upon and subject to the foregoing, we are of the opinion that the
Shares, when issued and delivered upon the exercise of options duly granted
pursuant to the Plan and against the payment of the purchase price therefor,
will be validly issued, fully paid, and non-assessable.
We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement.
Very truly yours,
/s/ Bingham Dana LLP
BINGHAM DANA LLP
